The present invention relates to an improved method for locating a fault (F) in a section of parallel transmission lines in a network by using symmetrical components.
Parallel series-compensated lines, i.e., lines provided with series capacitors and metal oxide varistors for improving the power transfer and enhancing power and voltage control of long transmission lines, are very important links between power generation and energy consumption regions. However, installation of series capacitors and varistors causes certain problems for fault location.
Accurate fault location in parallel power transmission lines with series compensation requires compensating for the following effects:
1. the remote infeed effect under resistive faults,
2. the effect of the mutual coupling between the lines for the zero sequence (xe2x80x94xe2x80x9czero sequencexe2x80x9d relates to symmetrical components in 3 phase A.C power transmission),
3. the effect of series compensation.
The countermeasures for the first two effects (1 and 2 as listed above) are applied for example in the single-ended fault locator proposed in U.S. Pat. No. 4,559,491. However, this fault locator is designated to locating faults in uncompensated lines, i.e., lines without series capacitor compensation. The cited method utilizes local measurements (phase voltages, post-fault and pre-fault phase currents from the faulted line and a zero sequence current from the healthy line) as well as the impedance parameters for the lines and for the equivalent supplying systems at both the line ends. As the remote system impedance is not measurable with the single ended method, the fault locator according to U.S. Pat. No. 4,559,491 uses an algorithm applying the representative value of this impedance for the positive sequence (xe2x80x94the term xe2x80x9cpositive sequencexe2x80x9d, and xe2x80x9cnegative sequencexe2x80x9d relate to symmetrical components in three phase A.C. transmission).
This is possible due to comparatively high robustness of this algorithm against mismatch of the actual and the representative values. However, in extreme cases of the high mismatch there is an additional source of error of the location algorithm. Moreover, the source impedances are also subjected to changes under evolving faults. In addition, if there is an extra link between the stations, the impedance of this equivalent link ought to be provided for the location algorithm, and obviously inaccuracy in this data affects the fault location too.
A countermeasure for the effect under point 3 above (the effect of series compensation) has been proposed in international patent application PCT/SE98/02404, where the idea from U.S. Pat. No. 4,559,491, as well as in Article xe2x80x9cA new fault locating algorithm for series compensated linesxe2x80x9d, IEEE Trensactions on Power Delivery, Vol. 14, No. 3, July 1999, pp 789-795, is extended to the case of locating faults in a single line with series compensation. For this purpose the fundamental frequency equivalenting of parallel branches of a compensating series capacitor (SC) and a Metal Oxide Varistor (MOV) has been introduced. Currents flowing through MOVs in particular phases during unsymmetrical faults have different amplitudes. As a consequence of that the parameters (resistances and reactances) of the fundamental frequency equivalents are different in particular phases. So, MOVs in particular phases may have different fundamental frequency representations in the fault location algorithm.
The object of the present invention is to provide a fault location method which does not require the knowledge of the source impedances of the system behind both stations and takes into consideration the reactance effect, the series compensation effect and the mutual coupling between the lines. The inventive fault location method generally comprises steps of measuring the currents and voltages of both lines at a measuring point arranged at one end (A) of the section, determining the fault distance (x) between the measuring point and the fault as a solution of an equation comprising the fault distance as a variable and the fault resistance.
The invention in its broad form resides in method for automatically locating a fault (F) in a section of parallel A.C. transmission lines in a network comprising the steps of:
measuring currents and voltages of both lines at a measuring point arranged at one end (A) of the section,
determining a fault distance (x) between the measuring point and the fault as a solution of an equation comprising the fault distance (x) as a variable and the fault resistance (Rf), wherein the equation is
Ax2xe2x88x92Bx+Cxe2x88x92Rf=0
wherein parameters A, B, C, D comprise phase components of the locally measured currents and voltages and are obtained by calculating from the measuring point to the fault location along both the parallel lines, and wherein the equation is resolved into its real and imaginary parts.
The method according to the present invention displays advantages in relation to the above mentioned prior art methods. It is suited for fault location in parallel lines with series compensation as well as after adequate setting for fault location in parallel uncompensated lines. It is based on phase coordinates approach which allows the incorporation of the fundamental frequency equivalents of the SCsandMOVs and to locate faults in the untransposed parallel lines. Further, it utilizes the local post-fault measurements, i.e., for the fault locator from the station A;xe2x80x94the phase voltages,xe2x80x94the phase currents from the faulted line andxe2x80x94the phase currents from the healthy line. It requires knowing the impedance parameters only for the lines (in terms of the self and mutual impedances of lines as well as for the mutual coupling between lines). Due to the utilization of the healthy line path, the impedances of the equivalent systems behind both the stations and the impedance of the equivalent link between the stations are not needed.
For determining the fault distance for series compensated lines, two subroutines are used: subroutine 1xe2x80x94estimating the distance to fault under assumption that it occurs behind the SCsandMOVs, and subroutine 2xe2x80x94estimating the distance to fault under assumption that it is applied in front of the SCsandMOVs. It is to be noted that usage of the algorithm for locating faults in parallel uncompensated lines relies on using only one of the subprocedures in which the fundamental frequency equivalents of the SCsandMOVs are set to zero for both the resistance and reactance parameters.
The sought fault distance is obtained with an extra selection procedure selecting the final result from the results of the both subroutines; the selection procedure yields the indication as to which subroutine is valid in a particular case on the base of information obtained:
a) the estimated fault resistances by both the subroutines,
b) amplitudes of the estimated currents of the healthy phases in the fault paths (for all the fault types except the three phase faults for which the selection procedure is performed only by (a)).
Thus, the present invention extends uncompensated lines fault locator to the case of parallel lines with series compensation, and extends the single series compensated line fault locator to the case of parallel series compensated lines. The main advantages of the present invention are that it does not require the knowledge of the source impedances of the systems behind both the stations and the impedance of the equivalent link between the stations. Moreover, it does not use the pre-fault measurements.
The introduced phase coordinates approach makes possible to incorporate the fundamental frequency equivalents into the model as well as to consider a line as an untransposed line.
These and other aspects of, and advantages with the present invention, will become apparent form the detailed description of an embodiment and from the accompanying drawings.